The present invention relates generally to optical disk storages having a focus servocontrol system in accordance with an astigmatism method.
An optical regenerative disk storage which reproduces a so-called compact disk (CD) has been well-known. In addition, a rewritable optical disk has also been well-known.
In a recording and/or reproducing apparatus (abbreviated optical disk storage hereinafter) used for such an optical disk, an optical beam focused by an objective lens emitted from optical beam generating means is radiated on a recording surface of the optical disk. The optical beam reflected on the recording surface of the optical disk is received by a reading sensor to reproduce data. The data may be recorded on the recording surface of the optical disk by radiating the optical beam thereon. In order for the desired recording/reproducing, it is necessary to very precisely focus an optical beam on the recording surface.
The optical beam is accessed at an arbitrary area on the recording surface of the rotating optical disk, and thus data is recorded and/or reproduced.
The recording surface of the optical disk vibrates upwardly and downwardly when the optical disk is rotated by the optical disk storage because of a deflection of the optical disk and an offset of a rotating shaft caused by a mechanical error generated when they are manufactured. It is necessary to move the optical beam generating means and the objective lens so as to make a focused optical beam follow the recording surface on the optical disk.
Such a control method in which the optical beam generating means and the objective lens are moved so as to follow the recording surface of the optical disk is one of the well-known focus servocontrol methods. The astigmatism method is among the focus servocontrol methods.
This astigmatism method uses a cylindrical lens, one surface of which is a plane and the other surface of which is a cylinder. A description of the characteristic of the optical beam passed into the lens will now be given. The optical beam having a circular section orthogonal to an optical axis, which beam has passed through a conventional lens, is focused after it passes through the cylindrical lens. The section orthogonal to the optical axis of the optical beam passed through the cylindrical lens has an elongated ellipse, in the vicinity of the cylindrical lens, which extends along a center shaft of the cylinder surface of the cylindrical lens. Then the section, which is apart from the cylindrical lens, gradually becomes a perfect circle and then becomes an ellipse in which the position of a long axis and a short axis are reversed.
The astigmatism method uses the above characteristic of the optical beam passed through the cylindrical lens by locating the cylindrical lens at a pupil point where the optical beam reflected on the recording surface of the optical disk is focused via the objective lens. Moreover, a photosensor used for the focus servocontrol is provided at a position where the section orthogonal to the optical axis of the optical beam becomes a perfect circle so that the optical beam passed through the objective lens can be focused on the recording surface of the optical disk.
Then, as the optical disk is rotated and the recording surface is vibrated, the optical beam focused via the objective lens is deviated from the recording surface toward the optical axis. The distance between the recording surface and the photosensor changes. On the other hand, the optical beam has reflected on the recording surface and passed through the objective lens and the cylindrical lens. Thus, the section of the optical beam on the photosensor is transformed from a perfect circle to an ellipse.
The photosensor used for the focus servocontrol comprises, as shown in FIG. 1A, four detecting parts a to d which are respectively made by dividing a quadrate into four equal parts. Each of these detecting parts a to d comprises a light receiving element for generating an output in proportion to the received light amount. The photosensor is located so that the center shaft of the optical beam passed through the cylindrical lens can correspond to an intersecting (center) point 0 of the respective detecting parts a to d.
If the optical beam focused via the objective lens is radiated on the recording surface of the optical disk, as shown in FIG. 1B, the optical beam has a perfectly circular section on the photosensor used for the focus servocontrol. Since the recording surface of the optical disk vibrates, if the recording surface moves upwardly and thus the distance between the optical beam generating means and the photosensor used for the focus servocontrol becomes longer, the optical beam has an elliptic section whose left side is biased as shown in FIG. 1C. On the other hand, if the recording surface moves downwardly, the optical beam has an elliptic section whose right side is biased, as shown in FIG. 1A.
A circuit connected subsequent to detecting parts a to d in the photosensor used for the focus servocontrol calculates a focus error by subtracting the sum of the outputs of the detecting parts a and c from the sum of the outputs of the detecting parts b and d. A control signal is generated based on this focus error.
The control signal becomes positive in the state shown in FIG. 1A, becomes zero in the state shown in FIG. 1B, and becomes negative in the state shown in FIG. 1C. In addition, the more the recording surface vibrates, the larger the difference between the long radius and the short radius of the ellipse shown in FIG. 1A or 1C becomes and thus the larger the value of the control signal becomes.
Thus, the control signal represents the detected vibration amount of the recording surface of the optical disk. The focus servosystem moves the optical beam generating means and the objective lens in accordance with the detected vibration amount. The position of the focused optical beam passed through the objective lens is adjusted so that the optical beam can be focused on the recording surface of the optical disk. That is, focus servocontrol is achieved.
Each mechanism in the optical disk storage is deflected due to the passing of time, surrounding temperature and change of humidity, etc. The center shaft of the optical beam radiated on the photosensor is gradually deviated from the center point 0, the intersecting point of the respective detecting parts a to d of the photosensor, even if it has initially corresponded thereto, because of the above causes, as shown in FIGS. 1D to 1F. The optical beam, which has been reflected on the recording surface and passed through the objective lens, has a perfectly circular section, as shown in FIGS. 1D to 1F, on the photosensor used for the focus servocontrol if the optical beam is precisely focused on the recording surface of the optical disk.
However, since the center of the circle on the radiated surface of the optical beam does not correspond to the intersecting point 0 of the respective detecting parts a to d, the difference between the sum of the outputs of the detecting parts a and c and that of the outputs of the detecting parts b and d, which should have been zero, cannot become zero. Thus the control signal includes an error. The control signal moves the objective lens upwardly and/or downwardly so as to compensate for the error of the control signal by assuming that the optical beam focused by the objective lens has been deviated from the recording surface. As a result, the focus of the optical beam on the recording surface, which originally corresponds to the recording surface, never corresponds thereto.
The ideal light amount distribution at the section orthogonal to the center shaft of the optical beam is equal to that on the concentric circle having the center corresponding to the center shaft. The ideal light amount distribution at the section orthogonal to the center shaft of the optical beam is deflected by mechanical deflection of each lens to control the generating and focusing of the optical beam, the deflection being caused due to the passing of time, temperature, change of humidity, etc.
Therefore, even if the center shaft of the optical beam corresponds to the intersecting point 0 of the respective detecting parts a to d, no focus offset is generated by the deflected light amount distribution of the section orthogonal to the center shaft of the optical beam and the optical beam has a perfectly circular section, each of the detecting parts a to d of the photosensor respectively receives a different light amount. Thus the control signal includes an error. A term "center error", as used herein, is a general term for an error of the biased optical amount distribution and that of the center shaft.
As a result, the center shaft of the optical beam radiated on the photosensor is deviated from the intersecting point 0 of the respective detecting parts a to d, so that the focus of the optical beam on the recording surface, which originally corresponds to the recording surface, never corresponds thereto.
Such a phenomenon makes the focused optical beam always deviate from the recording surface of the optical disk, and thus the optical energy radiated on the area to be accessed on the recording surface cannot reach the predetermined amount. Thus, recording/reproducing performed by the optical disk storage deteriorates.